Why do your cold water fish keep dying?

This question is rather difficult to answer, being that there is really nothing to go off of other than repeatedly dying fish. One of the fish probably introduced a viral, or bacterial, pathogen into the tank which has spread to the other tankmates and consiquentely caused them to die. Raise the salt concentration in the tank to 0.3% and treat the tank for parasites using an antiparasitic. It is also recommended to, if a bacterial pathogen is suspect, feed the fish an antibiotic food. If this does not help and all the fish die anyways you'll want to sterilize the tank before adding any new fish. You can do this using bleach as long as you let the tank sit for a couple days afterward.

i like the above answer however, i have an extensive knowledge of tropical fish-keeping and know about the benefits of using bleach as a cleaning agent & have thought about it but would never use it, to much risk involved.

cold water fish are for beginner's, i would recommend getting a water test by taking a clean bottle half full of your tank water to your local reputable fish shop.